Update docs to add image-builder user to /tmp

As noted in https://github.com/aws/eks-anywhere/issues/3368, we need to add additional documentation for machines where the `image-builder` user cannot access the `/tmp` directory in order to properly run the ubuntu image builds and create the ovas.

No documentation file or test is named in the issue. Start by locating the EKS Anywhere image-builder and Ubuntu image-build documentation, then verify the instructions cover machines where the image-builder user cannot access /tmp and explain how to create the OVAs successfully.

Written by the indexing model from the issue text.